The present invention relates to semiconductor power device, and more particularly relates to a semiconductor device which is formed of a wide bandgap semiconductor and in which a bi-directional switching operation can be performed and a sustaining circuit using the semiconductor device.
Semiconductor power devices are used in power electronic applications in which high voltage is applied and used for high current power switches of electronic instruments.
A known semiconductor power device such as a diode, a vertical-type MOSFET or the like has a pn juction. In this structure, current does not flow due to a depletion layer formed when a reverse bias is applied to the pn junction, so that the semiconductor device becomes resistant to high voltage. Because of this, when the known power device is operated as a switching device, it is necessary to convert an alternating voltage supplied from a power supply to a direct voltage so that the polarization of a voltage applied to the power device is kept constant.
As an example of such switching devices, a known vertical-type MOSFET will be described.
FIG. 6 is a cross-sectional view illustrating a general vertical-type MOSFET as an example of switching device. As shown in FIG. 6, the known vertical-type MOSFET includes an n-type Si (silicon) substrate 193, an n-type doped layer 192 formed on the principal surface of the Si substrate 193, a p-type well 195 formed so as to be surrounded by the n-type doped layer 192, an n-type source 196 formed so as to be surrounded by the p-type well 195, a gate insulation film 199 formed on a surface of the p-type well 195 having part sandwiched by the n-type doped layer 192 and the n-type source 196, a gate electrode 200 formed on the gate insulation film 199, a source electrode 197 formed on the n-type source 196, and a drain electrode 198 formed on the back surface of the Si substrate 193. In this structure, if the thickness of the Si substrate 193 is about 300 μm and the thickness of a silicon layer in which the n-type doped layer 192, the p-type well 195 and the n-type source 196 are formed is about 100 μm, a breakdown voltage of 1 kV can be ensured.
In the vertical-type MOSFET, electrons serve as carriers and a pn junction is formed between the n-type doped layer 192 and the p-type well 195. To operate the vertical-type MOSFET, a positive voltage is applied to the drain electrode 198 and the source electrode 197 is set at a ground potential. In this state, a positive voltage is applied to the gate electrode 200 to make a current flow through a channel, so that electrons flow from the n-type source 196 into a drain side. In this manner, the vertical-type MOSFET is turned ON. That is to say, by changing a gate voltage, ON and OFF of a current can be controlled. The vertical-type MOSFET allows precise control of electric instruments by an inverter or the like to contribute to reduction in power consumption. Note that switching devices include not only vertical-type MOSFETs but also IGBTs (insulated gate bipolar transistors) and the like.